Unexplained Sudden Weight Loss in Cats and Dogs - Vetstreet

Furthermore, all abnormalities detected should be treated concurrently to optimize clinical response to treatment. Many hyperthyroid cats are appropriately diagnosed and treated, but GI signs, especially weight loss, persist despite return to the euthyroid state. Subsequent evaluation of GI function as outlined above then reveals evidence of enteric disease and cobalamin deficiency. Only when these are also appropriately treated do the cats return to optimal health.

Weight loss caused by chronic disease in cats can result in additional symptoms:

Feline weight loss is the symptom that worries me the most of any warning signs you may see at home. It is the one cat illness symptom that most often has something serious associated with it.

Weight Loss Tips for Cats - The Conscious Cat

There are several options when it comes to adjusting your cat’s diet for weight loss. One flea swallowed can give your cat or kitten tapeworms. When the flea gets digested the tapeworm hatches. In order for a cat to lose weight with tapeworms s/he must likely has a large number of worms.
When I first got my cat she had a case of tapeworms. Above poster mentioned they look like grains of rice but once these dry up they look like sesame seeds. To this day nothing is allowed in my house with sesame seeds...buns, bagels...it just freaks me out thinking of when she had tapeworms.
You need to take your cat to a vet and find out the reason for the weight loss, only then can you give proper treatment. While at the vet get your cat on a flea preventive.

Weight Loss Programs for Overweight & Obese Cats - Pet Education

Your veterinarian will begin with a variety of diagnostic tests to find the underlying cause for your cat's weight loss. After an initial health assessment, your veterinarian may use one or more of the following tests:

Cat Weight Loss for Obese Cats - IAMS

The number of overweight and obese cats has exceeded to that of normal weighted cats in recent years. According to veterinarians, extra pounds on your cat lead to various disorders such as diabetes, arthritis and hepatic lipidosis. Most obvious reason which we can figure out for obesity in cats is that cats spend much time indoors and get less time for exercise. A very specific weight loss plan is required to help your cat in losing weight. Several positive changes in lifestyle and diet will be effective in helping cat to return to its pre-obese shape. Here’s how you can help your cat in cutting down extra pounds and stay fit:2.Select a weight loss program: Next task is to select a proper weight loss program. First comes diet, you have to feed your cat the food that is 20 % lower in calories than the foot you used to feed him earlier. Monitor the weight of your cat weekly after following the new diet. If the cat is losing weight accordingly, stick to the diet; otherwise reduce or increase the diet accordingly. Follow this until your cat has reached its target weight.